As the cultivated area per farm has increased heavily during the last decades and new areas have been cultivated, the need to collect stones from arable fields has strongly increased.
A lot of different models of stone pickers have been presented in the patent literature as far as from early 1900 century, the oldest ones American or Canadian powered by horses, later powered by tractors and even self-propelled models.
From the simple (frequently British) tractor carried solutions presented in the patent literature in the late -50es and early -60es the Japanese started with their trailed solutions in the -70es. The USA and Canada have also during this whole period a lot of patent applications. These solutions were frequently intended to collect smaller stones in swaths, or different models of loading buckets to collect stones (from the swaths), or to take up single big stones, for example when new land was taken in cultivation.
In the USSR a big number of patent applications and utility models have been presented from early -80es until the Soviet dissolution.
As a forerunner to the solutions, also to those seen today as modern, can rightfully be considered the publication FR2595185 from March 1986. In this publication the Frenchman Maurice Tanguy presents a stone picker pulled by a tractor and equipped with a lifting rotor, that co-operate with a sieve, the front end of which penetrates the ground surface, collecting stones and transporting them to a tilting container positioned at the rear end of the stone picker.
The throwing rotor of the Tanguy solution is mounted in a separate frame, movable in relation to the sieve to enable also bigger stones to pass without blocking the rotor.
Thus the publication FR2595185 presents the basic central solution, that is most commonly used in Europe even today, particularly if complemented with the solution presented in the patent publication CS263662 by the Czech Miroslav Svoboda and Co.
The publication CS263662 presents a stone picker combined with the already known aligned stone rakes, one on each side in front of the lifting rotor. In this manner, it was made possible to increase the working width of the machine and to save one separate working operation, that is raking the stones in a swath.
One can quickly convince oneself, that these solutions presented in the mid 1980es still are dominating the market by checking on the internet the offers of today, and the producers of stone pickers.
As an example we can from the KIVI-PEKKA STONE PICKERS leaflet of PEL-Tuote Oy, Seppälänsalmentie 181, FI 58900 Rantasalmi, see how the Kivi-Pekka™ stone picker is build using the teachings from the French and the Czech solutions from 1986.
It is also possible to find a fairly similar solution on the Kongskilde stone picker leaflet: STONEBEAR™ SB4000/SB5200 (101001044 EXP/GB/Stonebear 4000/BRO/1017).
Characteristically both the rotating stone rakes and the lifting rotor of a stone picker are mechanically driven from the tractor PTO [power takeoff] by use of drive shafts, gearboxes and V-belts, showing as well benefits as disadvantages.
The benefit with the mechanical transmission is that it is possible by use of quite simple and well-known components to transmit enough power and torque by normal load without disruption.
However, the load on a stone picker is strongly pulsating all the time, as a stone rake collects a flow of small and middle sized stones to the lifting rotor. The load shows additionally sharp load peaks as single big stones enter the machine. If the stones are oversized or unsuitable formed and enter the machine in “wrong edge”, blockage will occur.
Of course the mechanical transmission in known stone pickers is equipped with overload protections, but mostly these are really their Achilles' heel. Known types of overload protection used are multi disc friction clutches of standard type or, as in the above mentioned Kongskilde and PEL-Tuote machines, V-belt transmissions between the gearbox and the rotors.
The all-time pulsating load creates heating of the overload protections and as it is very difficult to equip these rotating components with an effective cooling. Therefor they easily are overheated and get unusable until their central components are replaced.
The drive torque for V-belts used as overload protection is extremely sensitive to dirt and moisture on the belts. If they got wet or dirty they have first to be cleaned and dried whereafter their pre-tension has to be checked and adjusted before the work can continue after an overload. That cleaning and the adjusting of the V-belts really is difficult, can clearly be understood thereby that for Kongskilde Stonebear™ the instructions for adjusting and cleaning the V-belts go over three pages in the instruction manual and include advise to grind the grooves in the pulleys when the machine is standing and to clean and dry the belts before the interrupted work can continue.
If the driver does not manage to disengage the PTO of the tractor immediately when overload occurs, these mechanical overload protections are very quickly overheated. The friction discs of the multidisc overload clutches get glazed and have to be replaced. The V-belts will burn and they also have to be replaced, adjusted and run in with due care during the run in period and thereafter readjusted checking the pre-tension rate during the run in time (if doubt, see the instruction manual of Stonebear™). All this additional work reduces the efficiency, gives expensive downtimes in the work and increases the risks concerning work safety.
Therefore the Canadian producer Degelman (Degelman Industries, 272 Industrial Drive, Regina, Saskatchewan S4P 3B1, Canada) for example, produces stone pickers equipped alternatively with mechanical or hydraulic transmissions. Check from Degelman Agricultural Equipment leaflet, the presentation of their model Signature™ 6000/7200.
The benefit of the mechanical transmission is that it can transmit high torques and the machine is therefore also able to collect big stones.
The easy reverse possibility for the collecting rotor, offered by the hydraulic transmission, is a big advantage during a blockage. Additionally the pressure relief valve of the hydraulic transmission gives a very reliable overload protection. The disadvantage is that the hydraulic capacity of most of the agricultural tractors normally available for this work, is not big enough with conventional solutions, to ensure the separate working units enough power to collect also big stones.